Effect of Cp-Ligand Methylation on Rhodium(III)-Catalyzed Annulations of Aromatic Carboxylic Acids with Alkynes: Synthesis of Isocoumarins and PAHs for Organic Light-Emitting Devices

Novel Metal-Free Synthesis of 3-Substituted Isocoumarins and Evaluation of Their Fluorescence Properties for Potential Applications

A novel metal-free synthesis of 3-substituted isocoumarins through a sequential O-acylation/Wittig reaction has been established. The readily accessible (2-carboxybenzyl)-triphenylphosphonium bromide and diverse chlorides produced various 1H-isochromen-1-one in the presence of triethylamine, employing sequential O-acylation and an intramolecular Wittig reaction of acid anhydride. Reactions using these facile conditions have exhibited high functional group tolerance and excellent yields (up to 90%). Moreover, the fluorescence properties of isocoumarin derivatives were evaluated at the theoretical and experimental levels to determine their potential application in fluorescent materials. These derivatives have good photoluminescence in THF with a large Stokes shift and an absolute fluorescence quantum yield of up to 14%.

Tandem C-H Annulation Reaction of Benzaldehydes and Aminobenzoic Acids with Two Equivalents of Alkyne toward Isocoumarin-Conjugated Isoquinolinium Salts: A Family of Organic Luminophores

A novel rhodium-catalyzed tandem C-H annulation of commercially available benzaldehydes and aminobenzoic acids with 2 equiv of alkyne is reported for the construction of isocoumarin-conjugated isoquinolinium salts that demonstrate diverse outstanding photoactivity. Depending on the substituents in the isoquinolinium moiety, they display either highly efficient fluorescence (up to 99% of quantum yield) or strong fluorescence quenching, which is provided by the transfer of the HOMO from the isoquinolinium to the isocoumarin moiety. Importantly, the functional groups in the benzaldehyde coupling partner also strongly affect the reaction selectivity, shifting the pathway to the formation of the photoinactive isocoumarin-substituted indenone imines and indenyl amines. Selective formation of the latter can be achieved by using a reduced amount of the oxidizing additive.

Triphenylcyclopentadienyl Rhodium Complexes in Catalytic C-H Annulations. Application for Synthesis of Natural Isocoumarins

Efficient protocols for the synthesis of triphenylcyclopentadienyl rhodium halides [(1,2,4-C₅Ph₃H₂)RhX₂]₂ (1a,b: X = Cl, I) starting from 1,2,4-triphenylcyclopentadiene or the cyclooctadiene derivative (1,2,4-C₅Ph₃H₂)Rh(cod) (2) were developed. Iodide abstraction from 1b with thallium or silver salts allowed us to prepare rhodocenium [(1,2,4-C₅Ph₃H₂)RhCp]PF₆ (3PF₆) and mesitylene complex [(1,2,4-C₅Ph₃H₂)Rh(mesitylene)](SbF₆)₂ (4(SbF₆)₂). Halides 1a,b (at 0.5 mol % loading) showed high catalytic activity in the construction of C-C, C-O, and C-N bonds via the C(sp²)-H activation approach. Their efficiency was demonstrated in the synthesis of more than 40 examples of polycyclic organic compounds (such as isocoumarins and naphthalenes, as well as isoquinolinium and dibenzo[a,f]quinolizinium salts). The protocols developed tolerate a wide range of functional groups. In particular, they were successfully used for the atom- and step-economical synthesis of hydroxy-substituted isocoumarins, including the natural product oospalactone 7fe. The 6- or 8-hydroxy-substituted isocoumarins showed moderate antiproliferative activity against several human cell lines in vitro.

Substituted Isocoumarins: An Assemble of Synthetic Strategies Towards 3-Substituted and 3,4-Disubstituted Isocoumarins

The versatility of isocoumarin frameworks offers the privilege to access many pharmacological targets. This unique heterocycle core present in many natural products and complex organic molecules contribute to medicinal chemistry as anti-cancer, anti-inflammatory and immunomodulatory agents. The attractive properties exhibited by its analogues urged the scientists to explore their synthetic analogues. In regard to the myriads of synthetic methodologies, we have compiled a review update covering all the articles that have been published towards the synthesis of 3-substituted and 3,4-disubstituted isocoumarins. Additionally, we have also highlighted a systematic survey of catalytic methods for their synthesis along with their scope towards diverse functionalizations and plausible mechanistic aspects.

Ru(ii)-catalysed oxidative (4 + 2) annulation of chromene and coumarin carboxylic acids with alkynes/propargylic alcohols: isolation of Ru(0) complexes

Ru(ii)-catalysed oxidative (4 + 2) annulation of chromene and coumarin carboxylic acids with alkynes affords pyrano-chromones via vinylic C–H bond activation; use of methyl-tethered propargylic alcohols instead of alkynes gives Ru(0) complexes.

An asymmetric metal-templated route to amino acids with an isoquinolone core via a Rh(III)-catalyzed coupling of aryl hydroxamates with chiral propargylglycine Ni(II) complexes

A general protocol for the asymmetric synthesis of artificial amino acids (AAs) comprising an isoquinolone skeleton was successfully elaborated via a straightforward Rh(III)-catalyzed C-H activation/annulation of various aryl hydroxamates with a series of robust chiral propargylglycine Ni(II) complexes derived from glycine (Gly), alanine (Ala) and phenylalanine (Phe) in a green solvent (methanol) under mild conditions (at room temperature under air). Notably, in the case of phenylalanine-derived complexes, the formation of unfavorable 4-substituted isoquinolone regioisomers was achieved by a catalyst control for the first time. The subsequent acidic decomposition of the obtained Ni(II) complexes provides the target unnatural α- and α,α-disubstituted AAs with an isoquinolone core in an enantiopure form.

Redirecting of Charge Transfer Enables the Control of the Photoactivity of Terarylenes

Photoinduced charge transfer affects the efficiency and selectivity of photochemical reactions. Incorporation of donating groups into the isoquinolinium core allowed us to overcome the photochemical inactivity of the corresponding dithienyl-substituted terarylenes, presumably by redirecting the charge transfer within the molecule, and gave access to a new family of thermally reversible photoswitches.

Easy Access to Versatile Catalytic Systems for C-H Activation and Reductive Amination Based on Tetrahydrofluorenyl Rhodium(III) Complexes

On the basis of the 1,2,3,4-tetrahydrofluorenyl ligand, a simple approach was developed to new effective rhodium catalysts for the construction of C-C and C-N bonds. The halide compounds [(η⁵ -tetrahydrofluorenyl)RhX₂ ]₂ (2 a: X=Br; 2 b: X=I) were synthesized by treatment of the bis(ethylene) derivative (η⁵ -tetrahydrofluorenyl)Rh(C₂ H₄ )₂ (1 a) with halogens. An analogous reaction of the cyclooctadiene complex (η⁵ -tetrahydrofluorenyl)Rh(cod) (1 b) with I₂ is complicated by the side formation of [(cod)RhI]₂ . The reaction of 2 b with 2,2'-bipyridyl leads to cation [(η⁵ -tetrahydrofluorenyl)Rh(2,2'-bipyridyl)I]⁺ (3). The halide abstraction from 2 a,b with thallium or silver salts allowed us to prepare sandwich compounds with incoming cyclopentadienyl, dicarbollide and mesityleneligands [(η⁵ -tetrahydrofluorenyl)RhCp]⁺ (4), (η⁵ -tetrahydrofluorenyl)Rh(η-7,8-C₂ B₉ H₁₁ ) (5), and [(η⁵ -tetrahydrofluorenyl)Rh(η-mesitylene)]²⁺ (6). The structures of 1 b, 2 b ⋅ 2I₂ , 3PF₆ , 4TlI₄ , 5, and [(cod)RhI]₂ were determined by X-ray diffraction. Compounds 2 a,b efficiently catalyze the oxidative coupling of benzoic acids with alkynes to selectively give isocoumarins or naphthalenes, depending on the reaction temperature. Moreover, they showed moderate catalytic activity in other annulations of alkynes with aromatic compounds (such as benzamide, acetanilide, etc.) which proceed through CH activation. Compound 2 b also effectively catalyzes the reductive amination of aldehydes and ketones in the presence of carbon monoxide and water via water-gas shift reaction, giving amines in high yields (67-99 %).